1. Field of Endeavor
The invention relates to a device on a shroud, which is provided on a turbine rotor blade tip, with a sealing structure designed like a line of ribs, locally projecting over the shroud radially to the rotational axis around which the turbine rotor blade rotates, which sealing structure has a longitudinal extent oriented in the circumferential direction of the turbine rotor blade, tapers with increasing radial distance from the shroud and has an end face which is formed flat and radially faces away from the turbine rotor blade.
2. Brief Description of the Related Art
Turbine rotor blades in most cases are provided with a shroud on their turbine rotor blade tips, which develops a vibration-reducing effect upon the respective turbine rotor blade airfoil and therefore promotes extension of the service life of the turbine rotor blade. Moreover, provision is made on each of the shrouds for at least one sealing structure of rib-like design which, on the end-face side, radially projects over the end-face shroud surface in relation to the rotational axis and extends along the shroud in the rotational direction in relation to the rotational movement of the turbine rotor blades.
Such sealing structures above all serve for reducing leakage flows which develop along the flow passage between the turbine rotor blade tips and the stationary turbine casing and which do not contribute to the power gain of the turbine. These sealing structures are based on abrasive materials and, as a result of rotation of the turbine rotor blades and on account of their radial prominence in relation to the shrouds, make it possible for an abradable wall structure, lying radially opposite the turbine rotor blade tips on the turbine casing and typically designed in the manner of a honeycomb structure, to be ground into, forming a circumferentially extending groove-like recess in such a way that the end-face shroud surface certainly includes a minimum gap with the wall structure, but the rib-like sealing element projects almost in an accurately fitting manner into the groove-like recess which is automatically cut out by the seal element. In axial projection, therefore, each rotor blade tip, with its sealing structure which engages in the groove-like recess, terminates in a largely gas-tight manner for a gaseous operating medium which flows axially through the turbine.
Vibration trials carried out on turbine rotor blades, however, showed that an almost total prevention of any leakage flows leads to strongly pronounced vibration instabilities along the rotating turbine rotor blade airfoils.
Such vibrations can be significantly reduced, however, if a leakage flow can develop between the turbine rotor blade tips and the turbine casing.
Therefore, it is necessary, in accordance with a ratio, which is as balanced as possible, between both phenomena which are in competition with each other, to seek to minimize the loss-affected leakage flows on the one hand and the occurrence of structure-weakening vibrations on the other hand.
For this, on the rib-like sealing structure which projects radially over the shroud and in the longitudinal extent typically has a largely constant cross-sectional shape, provision is made for a cutting structure which locally increases the cross-sectional shape axially, that is to say transversely, to the direction of rotation, and which on the two axially oppositely disposed flanks has cutting surfaces facing the sealing structure in the direction of rotation. The cutting surfaces, which project locally from the sealing structure, enable a wider dimensioned groove-like recess to be impressed, in comparison to the remaining axial sealing structure width, inside the abradable turbine casing wall which in most cases is formed as a honeycomb structure, so that the sealing structure is not able to lie in an accurately fitting manner over its entire longitudinal extent in the groove-like recess and therefore a leakage flow, which can be proportionally established, can develop as a result of the ensuing gap between sealing structure and groove-like recess.
It should be added that the rib-like sealing structure, which is provided on the shroud, in most cases does not coincide with the radial center of gravity plane of the turbine rotor blade in the radial direction along the turbine rotor blade, as a result of which additional load moments occur, especially at high speeds and high process temperatures, which can lead to increased creep rates and ultimately to material failure in the connecting region between the shroud and the turbine rotor blade airfoil.
For combating this load problem, it has been proposed in EP 1 507 066 A2 to arrange the cutting structures, which are provided on the rib-like sealing structure, largely centrally to the longitudinal axis of the rib-like sealing structure, wherein the cutting structure should lie as close as possible to the radial center of gravity position of the turbine rotor blade. In FIG. 6 of the printed version of EP 1 507 066 A2, a radial plan view of the rib-like sealing structure relative to the shroud is shown, which sealing structure has a V-shaped taper in cross section, with increasing radial distance to the shroud, and on the side flanks which face each other in each case has a cutting surface which is raised beyond the respective side flank, which cutting surfaces in longitudinal extent occupy different mutually offset positions in relation to the rib-like structure. The combination which can be gathered from EP 1 507 066 A2 and consists of a rib-like sealing structure which radially projects over the shroud of a turbine rotor blade, and a cutting structure which is attached to the sealing structure and located as close as possible in the region of the radial center of gravity line of the turbine rotor blade, enables the operation-induced vibration behavior and the material loads associated therewith, especially in the region of the rotor blade tip, to be positively influenced only in the case of turbine rotor blades up to a specific maximum size. If, however, the necessity should be for turbine rotor blades which are longer and therefore of larger construction, the shrouds of which assume a considerable length dimension oriented in the direction of rotation, virtually corresponding to that of the rib-like sealing structure which is oriented in the direction of rotation and spans the shroud similar to a bridge or an arch, then substantial bending loads occur in the radial direction, leading to high mechanical loads in the shroud region. In order to withstand these undesirably high bending loads, it would make sense in any event to enlarge the rib-like sealing structure according to EP 1 507 066 A2, that is to say to increase the height and width. Such a measure, however, leads to a significant mass increase and also to deterioration of the grind-in properties of the cutting contours which are provided along the rib-like sealing structure.